Such a wet classification device is known. It is used for the separation of the coarse portion of powders, as titanium dioxide, for example. So, today more and more ultrafine materials having a particle size between 0.1–5 pm are used. Such particles sizes can be obtained by fine grinding of crude substances which, however, implies relatively high operating costs since the energy consumption is very high, cooling systems are required, a relatively high environmental contamination is caused and a large portion of undersized particles is generated, etc. Such problems do not exist with wet classification centrifuges. Here, the solid (powder) is mixed with a liquid (normally water) in a pre-mixing tank in order to obtain a suspension. If necessary, the suspension is diluted in a following dilution tank and then introduced into the wet classification device which is formed as a centrifuge. Within the rotating centrifugal drum the heavier and coarser particles are fed radially outwardly against the drum wall in a faster manner than the lighter and finer particles of the suspension so that they are deposited as sediment. The sediment is removed from the drum wall in cleaning phases of the wet classification device wherein for this, depending on the respective embodiment, cleaning liquids, mechanical cleaning means, knives etc., are used. Finally, the sediment which is redispersed in the cleaning liquid is discharged from the centrifugal drum and is fed to a separately arranged ball mill, for instance, in which the coarse portion of the redispersed suspension is ground. Then, the process suspension can be introduced into the system again, for instance fed to the pre-mixing tank.
During the classification process the classified suspension is continuously withdrawn from the wet classification device and is made available for further usage.
When centrifuging ultrafine materials in a plurality of application cases high centrifuging factors are required for a high separation effect in addition to an optimum rotor design. On account of these high centrifuging factors a very stable and very hard sediment cake is generated with a corresponding centrifuging time. Furthermore, in a plurality of applications hard substances having a Mohs hardness between 3 to 10 have to be centrifuged while suspended in liquid. Normally, the devices known for scrubbing centrifuges fail in this connection, i.e. they cannot peel out or strip the sediment cake. This was confirmed in the past by worn knives, broken peeling devices as well as bent or broken knife shafts. Accordingly, on account of the high cutting forces also a braking of the main drive motor to standstill occurs.